This invention relates to a weighing apparatus comprising a number of grippers each designed for receiving therein an object to be weighed, each gripper being mounted on a conveyor via a connection, so that the grippers are arranged for movement along a gripper path, each gripper comprising a first guide element which, over at least a part of the gripper path, moves over a weighing surface of at least one weighing element, whereby the weight of the gripper and the object located therein is transferred via the first guide element to the at least one weighing element.
Such an apparatus is known from U.S. Pat. No. 5,703,332. The drawback of this known apparatus is that the connection between each gripper and the conveyor is formed by an assembly of at least two parallel rods which, on one side, are each connected with the gripper through a hinge and, on the other, are each connected with the conveyor through a hinge. For the purpose for which the known apparatus is intended, viz. weighing fruits such as apples, paprikas and the like, the accuracy of the known apparatus is sufficient. For some applications, however, a higher accuracy is required. In particular, a higher accuracy is required for weighing eggs with greater accuracy. In the known apparatus, a weighing error of xc2x11.5 grams must be taken into account. The invention contemplates a weighing apparatus in which a weighing error of about xc2x10.2 grams at a maximum occurs. The weighing error in the known apparatus is caused in particular by the friction in the hinges of the parallel rods. Further, the path followed by the conveyor, and hence the paths followed by the grippers, is not defined very accurately, owing to play and dimensional variations in the conveyor. As a consequence of the above-mentioned inaccuracy of the path followed by the conveyor, it happens regularly that the parallel rods do not assume a horizontal position when the first guide element moves over the weighing surface of the weighing element. Consequently, the force exerted by the conveyor on the gripper for the purpose of conveyance will result in a vertical force component, which is taken up by the weighing element, so that the measurement is undesirably influenced.
The invention contemplates a weighing apparatus of the type described above without the above-described disadvantages, that is, an apparatus capable of performing a highly accurate weighing while the objects are being conveyed.
To that end, according to the invention, the apparatus of the type mentioned in the preamble is characterized in that the connection between each gripper and the conveyor comprises at least one assembly of flexible members each having a particularly low spring constant in a direction perpendicular to the at least one weighing surface.
Because the connection between the conveyor and the gripper is formed by at least one assembly of flexible members each having a particularly low spring constant in a direction perpendicular to the at least one weighing surfacexe2x80x94a spring constant of less than about 0.1 Nmm can be considered herexe2x80x94the connection can be made of hingeless design. As a consequence of this hingeless connection, the weighing cannot sustain any influence from frictional forces arising in hinges, such as occur in the weighing apparatus according to the prior art. As a consequence of the absence of hinges and frictional forces arising therein, a particularly accurate weighing can be realized, an accuracy of about +0.1 gram being feasible.
Preferably, the first guide element and the or each weighing element are arranged above an object received in the gripper in question. This prevents dirt transported along with the object from ending up on the weighing element or the guide element.
A first embodiment of the weighing apparatus according to the invention is characterized by one assembly of flexible members arranged relative to the first guide element such that these members, at least when the first guide element moves over a said weighing surface, are all subject to tensile loading. In practice, the assembly of flexible members is designed as three flexible strips or leaf springs which are arranged in a triangle configuration, with the guide element disposed within this triangle, so that when the guide element sustains a frictional force in that it moves over the weighing surface, all three strips are subject to pull. As a consequence of such a design, the gripper maintains a stable position also during weighing.
A second embodiment of the weighing apparatus according to the invention is characterized by a second guide element and a reference guide over which the second guide element moves, at least when the first guide element moves over a said weighing surface, while a connection point between the conveyor and at least that flexible member that is located closest to the first guide element is displaceable in a direction perpendicular to the at least one weighing surface, the position of the displaceable connection point being controlled by cooperation of the second guide element and the reference guide, such that at least the or each flexible member provided with a displaceable connection point extends substantially parallel to the at least one weighing surface when the first guide element moves over a said weighing surface.
As a result of the fact that at least the flexible member that is located closest to the first guide element extends parallel to the weighing surface during weighing, that flexible member which is subject to the most tensile force will not transmit any vertical force component onto the weighing surface. Displacement of the gripper under the influence of play and dimensional inaccuracy in the transport system in a direction perpendicular to the weighing surface does not have any influence on the weighing, due to the displaceably arranged connection point, so that a very high weighing accuracy can be accomplished.
A third embodiment of the weighing apparatus according to the invention is characterized in that the connection between each gripper and the conveyor comprises a first and a second assembly of members, the weighing apparatus comprising a second guide element and a reference guide over which the second guide element moves, at least when the first guide element moves over a said weighing surface, the first assembly forming a connection between the second guide element and the conveyor, the second assembly forming a connection between the second guide element and the gripper, the members of the second assembly being flexible members each having a particularly low spring constant in a direction perpendicular to the weighing surface, the first and the second guide element and the at least one weighing surface and the reference guide being arranged relative to each other such that the flexible members of the second assembly extend substantially parallel to the at least one weighing surface when the first guide element moves over a said weighing surface.
In a weighing apparatus of such design, too, the conveyor does not exert any vertical force component of the conveying force on the weighing surface, in that the flexible members of the second assembly extend substantially parallel to the weighing surface. The weighing apparatus according to the third embodiment therefore has a particularly high accuracy. The members of the first assembly can be designed as flexible members. In an alternative further elaboration, the members of the second assembly can be designed as rods having their ends pivotally connected to the conveyor on the one hand and an intermediate piece carrying the second guide element.
Further elaborations of the invention are described below and will be further clarified hereinafter on the basis of three exemplary embodiments, with reference to the drawings.
It is noted that U.S. Pat. No. 3,980,147 discloses a weighing apparatus which comprises an assembly of flexible members. However, this is not a weighing apparatus which involves weighing of the objects while they are being conveyed. The problems for which the invention provides the solution, that is, minimizing the influence of conveying forces on weighing, does not play any role in the apparatus known from the above identified U.S. patent.